Hilary Duff: All Fours Vacay Triggers Milf Alert!

Hilary Duff's dogging it on the beach ... getting down on all fours and that adds up to a perfect 10 -- scientifically speaking.

We got these shots of the now officially single mom on vacation, soaking up the sand and surf on Maui.

Former NHL star Mike Comrie, aka Hil's new ex-husband, was also there with their 3-year-old son Luca. But this photo gallery is just for mom.